Physical Properties of the HDG Coating
The superior corrosion protection provided by hot-dip galvanizing is the most important reason for specifying zinc coatings. However, some other factors have an impact on the appearance and use of galvanized steels.
Galvanized coatings are generally bright and shiny but within a year will weather to a uniform dull gray appearance. The basic finish requirements of the galvanized coating are that it be relatively smooth, continuous, and free from gross surface imperfections. Smoothness is an ambiguous term; the product’s end use must be the determining factor in setting tolerances for smoothness. The galvanized coating is continuous and provides optimum corrosion protection.
Handling techniques for galvanizing may require the use of chain slings, wire or other holding devices to lower material into the galvanizing kettle if suitable lifting features are not available on the item. Chains, wires, and special jigs used to handle the items may leave a mark on the galvanized item. These marks are not detrimental to the coating, nor are they cause for rejection. If considered necessary, or expose bare steel, these areas can be easily touched up using the procedures described in ASTM A 780.
Differences in the luster and color of galvanized coatings do not significantly affect corrosion resistance. The presence or absence of spangle has no affect on coating performance. The well-known spangled effect found on galvanized products is a crystallization process that is dependent upon the zinc bath chemistry, the rate of cooling, the method of pickling, the steel chemistry, and the thickness of the coating. Dull gray or patchy matte gray galvanized coatings give a service life equal to bright or spangled coatings since the service life depends on the zinc coating thickness. Variations in coating appearance or finish are important only to the extent that they will affect the intended use of the article. The primary function of the galvanized coating is corrosion protection.
Abrasion Resistance/Resistance to Mechanical Damage
Other coatings damage easily during shipment or through rough handling on the job site. Experts will argue that all organic forms of barrier protection (such as paint) by their nature are permeable to some degree. Galvanized coatings are impermeable. Furthermore, if the galvanized coating is physically damaged, it will continue to provide cathodic protection to the exposed steel. If individual areas of underlying steel or iron become exposed by as much as 1/4” in diameter, the surrounding zinc will provide these areas with cathodic protection for as long as the coating lasts.
Figure 1 below shows how corrosion will begin and immediately progress at a scratch or gap in a paint coating. Figure 2 shows how corrosion will be prevented at a scratch or gap in a zinc coating.
Corrosion Protection and the Zinc Patina
Freshly galvanized steel progresses through a natural weathering process. During the first few weeks after an article has been galvanized, it develops a natural protective patina. If allowed to develop properly, the patina itself provides a corrosion protection layer for the active zinc metal.
The formation of the zinc patina begins with the development of a thin layer of zinc oxide particulates on the freshly coated surface. These particulates react with water, from rainfall or dew, to form a porous, gelatinous zinc hydroxide. During drying, this product reacts with carbon dioxide present in the atmosphere and converts into a thin, compact and tightly adherent layer of corrosion products consisting mainly of basic zinc carbonate. The rate of patina formation varies according to the environmental conditions.
Typically, it takes approximately 6-12 months to fully develop.
Handling and storage conditions can inhibit the formation of the patina. Storage areas that are high in humidity and lack air circulation tend to promote excessive growth of zinc oxide and zinc hydroxide. Adequate ventilation must be provided so that the build-up and retention of excessive water on the surface of the galvanized steel are avoided.
High Temperature Exposure
There are some concerns with using hot-dip galvanized steel in an elevated temperature environment. The industry has recommended the service temperature for conventional coatings to be less than 390ºF (200ºC) for long-term exposure. The problems that arise from long-term use at temperatures above 390ºF (200ºC) include peeling, some changes in mechanical properties, and obvious reduction in corrosion protection.
In an air environment, conventional galvanized coatings may withstand long-term continuous exposure of up to several months at 390ºF (200ºC) without separation of the outer zinc layer. The incidence of such failure will depend on the micro-structural characteristics of the coating. Higher temperatures progressively accelerate the separation process to the point where it requires only a few hours at 570ºF (300ºC) to cause peeling.
As discussed, peeling does occur at temperatures above 390ºF (200ºC) and is dependent on the rise in temperature and the duration exposed. But this does not mean that there isn’t any corrosion protection. During peeling, only the outer free zinc layer has become detached, leaving the zinc-iron alloy layers to provide corrosion protection to the steel.
The bright shiny appearance of a newly galvanized coating is sometimes not desired depending on the application. Some government agencies require that the surface of coated steel products not exceed certain reflectivity values. In the early 1970s there were four separate power companies/agencies that demanded galvanized products be dulled, that is, their reflectivity needed to be reduced to a range of 12% to 18% before field installation.
A newly-galvanized part with the normal pure zinc layer on the outside has a natural reflectivity of over 70% in the visible range. If the outer surface of the part has an intermetallic layer, rather than the normal pure zinc layer, the reflectivity is greatly reduced. This reflectivity change will also happen over time as the galvanized part weathers and the zinc oxides and carbonate films form on the surface of the part. However, if the surface must be dulled immediately, there have been some treatments used in the past that have proven successful. If surface reflectivity is an issue, contact your local galvanizer to discuss possible options to producing the desired coating appearance.